It is the goal for many commercial applications to improve the quality of thin magnetic layers that may be used as recording media for various technologies including heat assisted magnetic recording (HAMR) devices, magnetic random access memory (MRAM) and other memory or recording technology. In particular, a central challenge for present day magnetic recording is to increase the storage density in a given magnetic medium/magnetic memory technology. Several features of magnetic materials place challenges on density scaling for magnetic media. For one, memory density may be limited by the grain size of the magnetic layer, which is related to the magnetic domain size and therefore the minimum size for storing a bit of information. Secondly, the ability to read and write data in a magnetic layer is affected by the magnetocrystalline anisotropy of the material. In some cases, it may be desirable to align the easy axis of the magnetic material along a predetermined direction, such as along a perpendicular to the film plane for perpendicular memory applications.
Recently, magnetic alloys, and in particular, CoPt, CoPd, and FePt films have shown promise for high density magnetic storage. In particular, CoPt, CoFe, FePt and related materials form a tetragonal “L10” phase having high magnetocrystalline anisotropy and exhibiting the ability to form small crystallite (grain) size, both desirable features for high density magnetic storage. The L10 phase is believed to be the thermodynamically stable phase at room temperature for materials such as CoPt. However, when thin layers are prepared under typical conditions, such as being deposited by physical vapor deposition on unheated substrates, the face centered cubic (FCC) A1 phase is typically found. Preparation of the “L10” phase typically involves high temperature deposition of a thin film such as CoPt and/or high temperature post-deposition annealing, both of which may impact the ability to achieve the desired magnetic properties, and which may deleteriously affect other components of a magnetic device that are not designed for high temperature processing. Similarly, in the case of FePt films deposited at room temperature, the initial film structure is a disordered alloy A1 structure that requires annealing at about 500-600° C. to yield the ordered L10 face-centered-tetragonal (FCT) structure. Upon annealing, the grain size of such films may exceed desired limits for high density storage.
Recently, ion implantation of FePt was observed to reduce the amount of post deposition heat treatment required to form the L10 phase. By reducing the amount of thermal treatment required to form the desired L10 phase, the grain size may be maintained at a smaller level, thereby potentially increasing the storage density of magnetic media formed by such a process. However, for perpendicular magnetic data recording using materials such as L10 FePt, it is desirable to align the easy axis of the FCT phase along a desired direction to allow convenient reading and writing of data.
In this regard, conventional approaches suffer in that the microstructure of such L10 structures is less than ideal for high density storage. FIGS. 1a-1d depict an example of problems with the conventional approaches for forming the L10 phase. The coating material 102 is illustrated as deposited on a substrate 104, which may be any appropriate substrate. It is to be emphasized that the relative thickness of layers is not necessarily drawn to scale. For high density storage materials, such as perpendicular recording media, the layer thickness of such a coating material 102 may be below 100 nm and is some cases as thin as about 10 nm or less. Coatings may be deposited by vacuum deposition methods such as physical vapor deposition (PVD) as noted. As deposited, the coating material 102 is shown as having an FCC crystal structure in the close up view of FIG. 1a. In the FCC structure (also termed A1) for FePt, an iron atom may occupy any site of the FCC lattice as is also the case for platinum. The atoms of the material 102 are therefore represented by the same appearance. As noted, in prior art approaches, the use of heat treatment at temperatures in excess of 300° C. and typically in the range of 500-700° C. may result in the formation of the FCT phase as illustrated in FIGS. 1b to 1d. In particular, the coating material 102 is transformed into the coating material 110, which has the same overall composition as the coating material 102, such as FePt. However, the FCT phase is an ordered structure in which each Fe atom resides on a first set of lattice sites, while each Pt atom resides on a second set of lattice sites, such that the Pt atoms 112 arrange in planes of like atoms that are interleaved with planes of Fe atoms 114, as shown. In this L10 structure, the easy direction 116 of magnetization lies along the “c” axis of the FCT structure.
Although ion treatment may reduce the heat treatment or temperature of formation of the FCT phase having the L10 structure, in general, crystallites of FePt or other magnetic materials having the FCT L10 structure may assume any of multiple orientations after formation of the FCT phase. FIGS. 1b to 1d provide examples of different orientations that may be assumed by crystallites within a coating. The coating material of FIG. 1b, which is also denoted as coating material 110a to indicate a particular crystalline orientation, may represent one or more FCT crystallites formed from the coating material 102 having the FCC phase. As shown, coating material 110a exhibits an orientation in which the easy direction 116 is oriented perpendicular to the plane of the substrate 104, which is desirable for perpendicular storage applications. The coating material 110b of FIG. 1c exhibits an easy direction 116 that lies parallel to the plane of the substrate 104, which is less desirable for perpendicular storage. Finally, the coating material 110c of FIG. 2d has an easy direction 116 that forms a non-zero angle with respect to the plane of substrate 104, which is also less desirable for perpendicular storage.
Heretofore, apparatus and techniques are lacking to produce a microstructure in which the easy direction 116 of the L10 FePt is aligned along a perpendicular to the film, and in particular to perform such treatment at low temperature. Although the use of crystalline substrates such as MgO to promote epitaxial growth may be helpful, such approaches limit the flexibility of substrates for synthesizing magnetic layers and in any case may not result in formation of L10 FePt having the degree of easy axis alignment desired. Moreover, although magnetic fields have been applied to coatings, these fields are arranged within the plane of the substrate and are not well suited for aligning the easy axis perpendicular to the plane of the substrate. What is needed is an improved method and apparatus of forming perpendicular magnetic recording layers and devices.